1. Field of the Invention
The present invention relates to a digital radio communication system and method for switching a modulation scheme corresponding to a propagation environment, while using different modulation schemes between uplink and downlink channels.
2. Description of the Related Art
In a digital radio communication, a data transmission rate is increased as the modulation level of a modulation scheme is increased, however, using the multilevel modulation scheme in a poor propagation environment increases errors and degrades the signal quality.
As a method for improving both of the data transmission rate and quality, there is known a technique described in Technical Report of IEICE, RCS94-66. The technique switches modulation schemes such as QPSK, 16QAM, 64QAM, 256QAM and the like corresponding to a propagation environment.
In a digital radio communication system in which a base station performs radio communications with terminals, it is desired to improve a transmission rate on downlink used for the base station to transmit data to terminals, while when the benefit and convenience in the terminals are taken into account, reducing the scale and power consumption in the terminals is more desired than improving the transmission rate on uplink used for the terminals to transmit data to the base station.
Generally, as the modulation level of a modulation scheme is increased, power consumption in a power amplifier used to transmit signals is increased to obtain a predetermined quality. Therefore, in constructing a digital radio communication system, it is necessary to consider both of an improvement in the data transmission rate and the benefit and convenience in terminals.
However, at present, there exists no digital radio communication system that switches modulation schemes taking the benefit and convenience in terminals into account.